1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a transflective liquid crystal display (LCD) device implementing a color filter having various thickness.
2. Discussion of the Related Art
LCD devices are usually classified into transmission type and reflection type according to their difference in a light source.
The transmission type LCD device uses light incident from a back light that is attached to a rear surface of a liquid crystal panel. The light is incident to a liquid crystal layer of the liquid crystal panel, and is absorbed or passes through the liquid crystal layer according to proper alignments of the liquid crystal layer. The alignment of the liquid crystal layer can be controlled by way of controlling an electric field, which is applied to the liquid crystal layer. Therefore, a transmittance ratio of the liquid crystal panel can be controlled by way of applying the electric field to the liquid crystal layer. Conventionally, the back light of the transmission type LCD device is an artificial light source. Therefore, high power consumption due to the back light is a greater disadvantage of the transmission type LCD device.
On the contrary to the above-mentioned transmission type LCD device, the reflection type LCD device uses an ambient light incident from a natural light source or an exterior artificial light source. Because of its low power consumption, the reflection type LCD device is focused on. However, the reflection type LCD device is useless when the whether or exterior light source is dark.
Accordingly, a transflective LCD device is developed to compensate for the reflective type LCD device. The transflective LCD device is useful regardless of the whether or exterior light source. FIG. 1 is an exploded perspective view illustrating a typical transflective LCD device.
The transflective LCD device 1 includes upper and lower substrates 10 and 20 that are opposed with each other, and an interposed liquid crystal layer 50 therebetween. The upper and lower substrates 10 and 20 are called a color filter substrate and an array substrate, respectively. In the upper substrate 10, on a surface opposing the lower substrate 20, black matrix 12 and color filter layer 14 that includes a plurality of red (R), green (G), and blue (B) color filters are formed. That is to say, the black matrix 12 surrounds each color filter, in shape of an array matrix. Further on the upper substrate 10, a common electrode 16 is formed to cover the color filter layer 14 and the black matrix 12.
In the lower substrate 20, on a surface opposing the upper substrate 10, a TFT “T” as a switching device is formed in shape of an array matrix corresponding to the color filter layer 14. In addition, a plurality of crossing gate and data lines 26 and 28 are positioned such that each TFT is located near each cross point of the gate and data lines 26 and 28. Further on the lower substrate 20, a plurality of reflective electrodes 22 are formed on an area defined by the gate and data lines 26 and 28. The area there defined is called a pixel region “P.” Each reflective electrode 22 has a transmissive portion 22a thereon. The transmissive portion 22a beneficially has a shape of a through hole such that it exposes a transparent electrode 24 disposed below the reflective electrode 22. The reflective electrode 22 is beneficially made of a metal having a high reflectivity, and the transparent electrode 24 is beneficially made of a transparent conductive material, usually indium tin oxide (ITO) or indium zinc oxide (IZO).
FIG. 2 shows a cross-sectional view illustrating the transflective LCD device of FIG. 1. As shown, between the upper and lower substrates 10 and 20, a liquid crystal layer 50 is interposed. The upper substrate 10 has the color filter layer 14 and common electrode 16 on the inner surface opposing the lower substrate 20. On the common electrode 16, an upper alignment layer 142 is formed. In addition, on the exterior surface of the upper substrate 10, a retardation film or a half wave plate (HWP) 46 and an upper polarizer 54 are sequentially disposed. The half wave plate (HWP) 46 serves to involve a phase difference of “λ/2” for incident rays such that the incident rays rotate to have a phase difference of “λ/2” after passing through the half wave plate 46.
In the meanwhile, the lower substrate 20 has the reflective electrode 22 and transparent electrode 24 on its surface opposing the upper substrate 10. A lower alignment layer 44 is formed on the reflective electrode 22 and exposed portion of the transparent electrode 24. Between the reflective and transparent electrode 22 and 24; a passivation layer 48 is interposed to separate them. The reflective electrode 22 has the transmissive portion 22a, which exposes the transparent electrode 24. In addition, on the exterior surface of the lower substrate 20, a lower polarizer 52 is disposed, and below the lower polarizer 52, a back light 40 is disposed.
For forming the reflective and transparent electrode 22 and 24, at first, the transparent conductive material selected from indium-tin oxide (ITO) or indium zinc oxide (IZO) is deposited on the lower substrate 20. The transparent conductive layer is patterned to form the transparent electrode 24. Then, an insulating material is deposited on the transparent electrode 24 to form the passivation layer 48. On the passivation layer 48, aluminum (Al) based metal of a high reflectivity is deposited and patterned such that the reflective electrode 22 is formed. At this point, portions of the reflective electrode 22 and passivation layer 48 are sequentially etched away to form the transmissive portion 22a. 
The liquid crystal layer 50 between the upper and lower substrates 10 and 20 has an optical anisotropy. That is to say, in their first state alignment, long axes of the liquid crystal molecules are aligned parallel to the substrates 10 and 20. Whereas, with an electric field applied across the liquid crystal layer 50, the long axes of the molecules are aligned perpendicular to the substrates 10 and 20. Therefore, the liquid crystal layer 50 serves as a switch for incident rays of light. In the later state alignment, a homeotropic alignment, the rays pass through the liquid crystal layer 50, without a phase difference.
The liquid crystal layer 50 has a layer thickness or cell gap. Specifically, the liquid crystal layer 50 has a first cell gap “d1” over the reflective electrode 22 and a second cell gap “d2” over the transparent electrode 24. At this point, the first and second cell gaps “d1” and “d2” beneficially have a definite relationship. That is to say, the second cell gap d2 is beneficially twice as the first cell gap d1 (d2≈2d1). Over the reflective electrode 22, the liquid crystal layer 50 involves a phase difference of “λ/4.” The above-mentioned different cell gaps “d1” and “d2” improve en efficiency of incident rays passing through the transmissive portion 22a. 
More detailed explanation is followed with reference to relationships (1) and (2):d1Δn=λ/4  (1),d2=2d1  (2),                such that d2Δn=λ/2, wherein “d1” is the first cell gap over the reflective electrode 22, “d2” is the second cell gap over the transmissive portion 22a or transparent electrode 24. The first relationship (1) about the phase difference “λ/4” means that rays get the phase difference of “λ/4” after passing through the liquid crystal layer 50 of the first cell gap “d1” over the reflective electrode 22. Similarly, the relationship “d2Δn=λ/2” means that the rays get the phase difference of “λ/2” after passing through the liquid crystal layer 50 of the second cell gap “d2” over the transmissive portion 22a.         
Rays from the back light 40 pass through the lower polarizer 52 and are linearly polarized according to a first transmittance axis of the lower polarizer 52. That is to say, the lower polarizer 52 transmits only a portion of the incident rays that has a corresponding vibration direction parallel to the first transmittance axis of the lower polarizer 52. A vibration direction of rays is perpendicular to a travelling direction of the rays.
Then, the linearly polarized rays pass through the liquid crystal layer 50 over the transmissive portion 22a and get the phase difference of “λ/2”, which is explained above. At this point, the liquid crystal molecules are aligned in the first state alignment without an electric field applied thereto. The phase difference “λ/2” makes the rays rotate such that they have a vibration direction perpendicular to the first transmittance axis of the lower polarizer 52. After passing through the liquid crystal layer 50, the rays subsequently pass through the half wave plate (HWP) 46 and get the additional phase difference of “λ/2”, which means that the rays rotate to have a different vibration direction parallel to the first transmittance axis of the lower polarizer 52. At this point, the upper polarizer 54 has a second transmittance axis perpendicular to the first transmittance axis of the lower polarizer 52. Therefore, the rays passing through the half wave plate 46 are totally absorbed by the upper polarizer 54 such that a dark state for the transmissive portion 22a is achieved. Since the upper polarizer 54 absorbs all the rays, the dark state for the transmissive portion 22a is surely dark.
On the contrary, if the second cell gap d2 is equal to the first cell gap d1, rays passing through the liquid crystal layer 50 over the transmissive portion 22a get the phase difference of “λ/4” according to the first relationship (1), d1Δn=d2Δn=λ/4. That is to say, the rays are circularly polarized due to the phase difference “λ/4” of the liquid crystal layer 50. The circularly polarized rays subsequently pass through the half wave plate 46, and meet the upper polarizer 54. At this point, the circularly polarized rays include a parallel portion parallel to the second transmittance axis of the upper polarizer 54. Therefore, the parallel portion of the circularly polarized rays passes through the upper polarizer 54 such that the dark state has a gray level, which means that the dark state cannot be achieved.
Accordingly, the different cell gaps “d1” and “d2” are beneficially used for the clear dark state. With reference to FIGS. 3A, 3B, 4A, and 4B, operation modes for the typical transflective LCD device will be provided in more detail.
Phase changes of incident rays result from the operation of the upper and lower polarizers 54 and 52, liquid crystal layer 50, and half wave plate 46. Therefore, FIGS. 3A, 3B, 4A, and 4B refer to only the above-specified elements. In addition, as previously mentioned, the liquid crystal layer 50 has a homogeneous alignment at its first state alignment, and a homeotropic alignment with an electric field applied across the liquid crystal layer.
At first, FIG. 3A illustrates a dark state or mode for the transmissive portion 22a of FIG. 2. After rays of incident light from the back light 40 (see FIG. 2) pass through the lower polarizer 52, they are linearly polarized according to the first transmittance axis of the lower polarizer 52. At this point, the first transmittance axis has a direction of, for example, 45 degrees with respect to the long axis of the substrate 10 or 20 (see FIG. 1). Therefore, the linearly polarized rays passing through the lower polarizer 52 have the same vibration direction of 45 degrees as the first transmittance axis direction.
The linearly polarized rays subsequently pass through the liquid crystal layer 50 over the transmissive portion 22a. At this point, the liquid crystal layer 50 over the transmissive portion 22a is in the first state alignment with the second cell gap “d2.” Therefore, the liquid crystal layer 50 over the transmissive portion 22a involves the phase difference of “λ/2” such that the linearly polarized rays rotate to be perpendicular to the first transmittance axis of the lower polarizer 52. Then, the half wave plate 46 additionally involves the same phase difference of “λ/2” such that the linearly polarized rays rotate to be parallel to the first transmittance axis of the lower polarizer 52. Finally, the upper polarizer 54, which has the second transmittance axis perpendicular to the first transmittance axis, absorbs all of the linearly polarized rays parallel to the first transmittance axis of the lower polarizer 52. Accordingly, the dark state of the transmissive portion 22a is achieved.
On the contrary with FIG. 3A, FIG. 3B illustrates a white state for the transmissive portion 22a of FIG. 2. At this point, the liquid crystal layer 50 has the homeotropic alignment with an electric field applied across the liquid crystal layer 50. Therefore, the liquid crystal layer 50 involves an optically isotropic property, and no phase difference occurs due to the liquid crystal layer 50.
Rays from the back light 40 are linearly polarized after passing through the lower polarizer 52. Then, the linearly polarized rays pass through the liquid crystal layer 50 without phase change, and meet the half wave plate 46. The half wave plate 46 involves the phase difference λ/2 such that the linearly polarized rays are parallel to the second transmittance axis of the upper polarizer 54. The second transmittance axis has a direction of 135 degrees, for example. Therefore, the upper polarizer 54 transmits all the rays such that the white state of the transmissive portion 22a is achieved.
FIG. 4A illustrates a dark state for the reflective electrode 22 of FIG. 2. At this point, the liquid crystal layer 50 involves the phase difference of “λ/4”, and is aligned in the first state alignment, the homeotropic alignment with the second cell gap “d2.” At first, the upper polarizer 54 linearly polarizes rays of incident light from an external light source such that they have the same vibration direction of 135 degrees as the second transmittance axis of the upper polarizer 54. Then, the first linearly polarized rays pass through the half wave plate 46. The half wave plate 46 rotates the vibration direction of the rays such that the first linearly polarized rays have a vibration direction of 45 degrees.
Subsequently, the rays pass through the liquid crystal layer 50 over the reflective electrode 22 of FIG. 2. The liquid crystal layer 50 circularly polarizes the rays with the phase difference “λ/4” such that the rays change as left-circularly polarized (LCP) rays. Then, the reflective electrode 22 (see FIG. 2) below the liquid crystal layer 50 reflects the LCP rays such that the LCP rays reverses its phase and travelling direction to be right-circularly polarized (RCP) rays.
Then, the liquid crystal layer 50 involving the phase difference “λ/4” changes the RCP rays to second linearly polarized rays having a vibration direction of 135 degrees, which is parallel to the second transmittance axis of the upper polarizer 54. The second linearly polarized rays subsequently pass through the half wave plate 46 and rotate to be perpendicular to the second transmittance axis of the upper polarizer 54. Since the upper polarizer 54 absorbs all the rays, the dark state of the reflective electrode 22 of FIG. 2 is achieved. As shown in FIGS. 3A and 4A, when the liquid crystal layer 50 is in the first state alignment without applied electric field, the conventional transflective LCD device of FIGS. 1 and 2 provides the dark state.
On the contrary to FIG. 4A, FIG. 4B illustrates a white state for the reflective electrode 22 of FIG. 2. At this point, the liquid crystal layer 50 is in the homeotropic alignment, which involves no phase difference. At first, the upper polarizer 54 linearly polarizes incident rays from an external light source. The first linearly polarized rays passing through the upper polarizer 54 has the same vibration direction of 135 degrees as the second transmittance axis of the upper polarizer 54. The first linearly polarized rays subsequently pass through the half wave plate 46. The half wave plate 46 rotates the vibration direction of the rays such that the first linearly polarized rays have a vibration direction of 45 degrees.
Then the rays pass through the liquid crystal layer 50 without phase difference and meet the reflective electrode 22 of FIG. 2. The reflective electrode 22 of FIG. 2 reflects the rays such the rays turn to have the vibration direction of 135 degrees, which is perpendicular to the second transmittance axis of the upper polarizer 54. Subsequently, the rays meet the half wave plate 46. The half wave plate 46 rotates the vibration direction of the rays such that the rays have a vibration direction of 45 degrees again. Therefore, the upper polarizer 54 transmits all the rays such that the white state of the reflective electrode 22 of FIG. 2 is achieved. As shown in FIGS. 3B and 4B, when the liquid crystal layer 50 is in the homeotropic alignment, the conventional transflective LCD device of FIGS. 1 and 2 provides the white state.
In another aspect, a color property should be considered in designing the transflective LCD device. Conventionally, the reflective electrode 22 of FIG. 5 implements a better color purity property than the transmissive portion 22a. As shown in FIG. 5, for a transmissive mode of the transflective LCD device 1, a first incident light “A” from the back light 40 only once passes through the color filter layer 14 having thickness “t1.” However, for a reflective mode, a second incident light “B” from an exterior light source (not shown) twice passes through the color filter layer 14 having the same thickness “t1.” That is to say, in the transmissive mode, the first incident light “A” is only once colored by the color filter layer 14. Whereas, in the reflective mode, the second incident light “B” is twice colored by the color filter layer 14. Therefore, regardless of the difference in luminance of the different light sources, the reflective mode of the reflective LCD device implements a better color purity property than the transmissive mode thereof.
To overcome the above-mentioned problem, a dual color filter layer having different thickness is conventionally adopted for the transflective LCD device. FIG. 6 shows a typical reflective LCD device having the above-mentioned dual color filter layer. As shown, the conventional dual color filter layer 30 has first and second portions 30a and 30b according to their thickness and location. Between the dual color filter layer 30 and common electrode 16, a planar layer 90 is interposed. Specifically, the first portion 30a is positioned over the transmissive portion 22a and has a second thickness “t2” while the second portion 30b is positioned over the reflective electrode 22 and has a third thickness “t3.” The second thickness “t2” is beneficially greater than the third thickness “t3” such that an incident light from the back light 40 to the transmissive portion 22a takes more color purity in the transmissive mode. Consequently, the color purity property is uniform regardless of the different modes, the transmissive and reflective modes.
FIGS. 7A to 7D illustrate a fabricating process for the above-mentioned dual color filter layer 30. Generally, a typical color filter layer is formed on an upper substrate of a LCD device and includes red, green, and blue color resins “R”, “G”, and “B.” In addition, the color filter layer usually includes a black matrix (BM) formed between the color resins to shield incident light.
At first as shown in FIG. 7A, on a transparent insulating substrate or upper substrate 10 (reference upper substrate 10 of FIG. 6), chromium oxide (CrOx) and chromium (Cr) are sequentially deposited and patterned to form the black matrix 72. The black matrix 72 has a patterned shape corresponding to the color filter layers, which will be formed subsequently.
Since light can only be modulated at the area of the reflective electrode and transparent electrode 22 and 24 of FIG. 2, light passing through intervals between the reflective electrode and metal patterns (reference 28 and 26 of FIG. 1) degrade a display quality and should be eliminated. Therefore, the black matrix 72 is formed to cover the intervals. Further, the black matrix 72 shields an active area of a thin film transistor “T” (see FIG. 1) from light, unless electrical properties of the thin film transistor are deteriorated. For forming the black matrix 72, an assembly margin is considered.
Then, as shown in FIG. 7B, a red color resin is deposited and patterned on the substrate 10 to form the first red color filter layer 74. A photolithography process including an exposure step is used for forming the color filter layer. Since the color resin usually has a characteristic of a negative photoresist, non-exposed portions of the color resin are etched away. Then, green and blue color resins are sequentially deposited and patterned on the substrate 10 to respectively form the first green and blue color filter layers 76 and 78. Each of the first red, green, and blue color filter layer corresponds to one pixel region “P” shown in FIG. 1.
Thereafter, as shown in FIG. 7C, another red color resin is deposited and patterned on the first red color filter layer 74 such that a second red color filter layer 84 is formed. The photolithography process is also used for the second red color filter layer 84. At this point, the second red color filter layer 84 beneficially has the same area and location as the transmissive portion 22a shown in FIG. 6. Then, another green and blue color resins are sequentially deposited and patterned on the first green and blue color filter layers 76 and 78, respectively, such that second green and blue color filter layers 86 and 88 are formed.
Thereafter, as shown in FIG. 7D, a planar layer 90 is formed to cover the first and second color filter layers. The planar layer 90 is beneficially selected from an organic insulating material such as benzocyclobutene (BCB) and acryl resin, or an inorganic insulating material such-as silicon dioxide (SiO2) and silicon nitride (SiNX). The planar layer 90 serves to compensate the stepped shape of the first and second color filter layers such that a leveled surface is provided for the substrate 10. On the planar layer 90, a transparent conductive material such as indium tin oxide (ITO) and indium zinc oxide (IZO) is deposited to form the common electrode 16.
As explained above, the conventional fabricating method for the dual color filter layer needs at least six photolithography processes. Too many photolithography processes cause high material cost and low yield to the above-mentioned conventional fabricating method.